Energetics

1. Chapter 13
Enthalpy Changes
LEARNING OUTCOMES
 Distinguish between exothermic and
endothermic reactions in terms of energy
content of products and reactants
 Draw an energy profile diagram to illustrate
endothermic and exothermic change
 Calculate energy changes from experiments
or experimental data

2. Chapter 13
Enthalpy Changes
Exothermic reactions
 When a piece of magnesium ribbon burns
in air or oxygen, heat and light energy are
produced.
 This type of chemical reaction in which heat
energy is produced and given out to the
surroundings is called an exothermic change
or reaction.
An exothermic reaction is a reaction in which heat energy
is given out to the surroundings.

3. Chapter 13
Enthalpy Changes
Exothermic reactions
 During an exothermic reaction,
the temperature of the
surroundings rises since heat
energy is given out by the
reaction.

4. Chapter 13
Enthalpy Changes
Other exothermic reactions
 Freezing and condensation (physical changes)
 Combustion of fuels
 Neutralisation of acids and alkalis
 Dissolving solid sodium hydroxide, anhydrous sodium
carbonate and anhydrous copper(II) sulphate in water
 Adding water to concentrated sulphuric acid

5. Chapter 13
Enthalpy Changes
Endothermic reactions
When ammonium chloride crystals are dissolved in
water, there is a drop in temperature. This shows that
heat energy is absorbed from the surroundings.
An endothermic reaction is a
reaction in which heat energy is
absorbed from the surroundings.
In an endothermic reaction, the
temperature of the surroundings drops,
since heat energy is absorbed.

6. Chapter 13
Enthalpy Changes
Other Endothermic reactions
 Melting and boiling (physical changes)
 Thermal decomposition of compounds such as calcium
carbonate (limestone) and copper(II) carbonate
 Photosynthesis
 Taking a photograph with film (decomposition of
silver bromide into silver and bromine)
 Dissolving ionic crystals in water, e.g.
dissolving ammonium nitrate, ammonium chloride and
sodium carbonate crystals in water

7. Chapter 13
Enthalpy Changes
Heat of Reaction
 The amount of heat energy given out or taken in
during a chemical reaction is called the heat of reaction or
enthalpy change.
 Enthalpy change is represented by the symbol, ΔH.
 Matter contains both potential and kinetic energies. During
an exothermic reaction, heat is given out by the reactants to
the surroundings. Thus, enthalpy change is negative.
 During an endothermic reaction, heat energy is absorbed
by from the surroundings. Reactants gain energy, so
enthalpy change is positive.

8. Chapter 13
Heat of Reaction
 In an exothermic reaction, the
reactants are at a higher energy
level than that of the products.
 The excess heat energy is given
out by the reactants when the
products are formed.
 Since the reactants have lost
heat energy, we take ΔH as
negative.
Exothermic reaction
Enthalpy Changes

9. Chapter 13
Enthalpy Changes
Heat of Reaction
 In an endothermic reaction,
the reactants are at a lower
energy level than that of
the products.
 Heat energy is absorbed
by the reactants from the
surroundings in order to
change to the products. Endothermic reaction
 Since the reactants have gained
heat energy, we take ΔH as
positive.

10. Chapter 13
Enthalpy Changes
Heat of Reaction
 The heat of reaction can be included in a chemical
equation as:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) [ΔH = - 890 kJ]
 The negative value of ΔH means that the reaction is
exothermic.
 “890 kJ” means that when 1 mole of methane burns completely
in oxygen, 890 kJ of heat energy is given out.

11. Chapter 13
Enthalpy Changes
Heat of Reaction
CCaaCCOO33  CCaaOO ++ CCOO22 [[ΔΔ HH == ++ 222222 kkJJ]]
 The positive value of ΔH means that the reaction is
endothermic.
 “222 kJ” means that when 1 mole of calcium carbonate is
decomposed, 222 kJ of heat energy is absorbed.

12. Chapter 13
Enthalpy Changes
Bond breaking and bond making
 The reason why reactions are endothermic or exothermic
is because energy is taken in or given out when chemical
bonds are broken or made.
 Heat energy is absorbed to break bonds, so bond
breaking is endothermic.

13. Chapter 13
Enthalpy Changes
Bond breaking and bond making
 Heat energy is given out (released) when bonds
are formed, so bond forming is exothermic.

14. Chapter 13
Enthalpy Changes
Bond breaking and bond making
 So, the overall heat change in a reaction is given by:
Heat of reaction, ΔH = Heat absorbed Heat given out
in bond breaking in bond forming
 If more energy is absorbed for breaking bonds than
released in forming bonds, the reaction will be
endothermic.
 If more energy is released in forming bonds than
absorbed for breaking bonds, the reaction will be
exothermic.

15. Chapter 13
Enthalpy Changes
Worked example
Hydrogen burns in oxygen to form water according to the equation:
2H2(g) + O2(g) 2H2O (g)
By calculating ΔH, state whether the reaction will be endothermic or
exothermic, given the following bond energies per mole:
H-H = 436 kJ, O=O = 496 kJ, O-H = 463 kJ.
Solution:
H H O O
+ H
H
O
H H
H H O
Energy absorbed to break bonds = (2 x 436) + (1 x 496) = 1368 kJ
Energy released to form bonds = (4 x 463) = 1852 kJ
Hence, Δ H = Energy absorbed – Energy released
= (1368 – 1852) kJ = - 484 kJ
Since ΔH is negative, the overall reaction is exothermic.

16. Enthalpy Changes
Summary
Exothermic reaction Endothermic reaction
Heat energy is given out to the
surroundings.
Heat energy is absorbed from the
surroundings.
ΔH is negative. ΔH is positive.
More heat is given out in making
bonds than absorbed in breaking
bonds.
More heat is absorbed in breaking bonds
than given out in making bonds.
Chapter 13

17. Chapter 13
Enthalpy Changes
Quick check
1. What is meant by (a) an exothermic reaction, (b) endothermic reaction?
2. What is meant by enthalpy change? What symbol is used to represent enthalpy
change?
3. What type of reaction is it when the enthalpy change ΔH is
(a) positive, (b) negative ?
4. Sodium hydroxide reacts with nitric acid according to the equation:
NaOH(aq) + HNO3(aq)  NaNO3(aq) + H2O(l) ΔH = - 57.3kJ
(a) Is this reaction endothermic or exothermic? Explain your
answer.
(b) What would be the heat evolved if 2 moles of sodium
hydroxide are completely neutralised ?
5. State whether the following are exothermic or endothermic reactions:
(a) decomposing zinc carbonate,
(b) reacting potassium with water,
(c) forming hydrogen atoms from hydrogen molecules,
(d) forming chlorine molecules from chlorine atoms,
(e) respiration,
(f) photosynthesis. Solution

18. Chapter 13
Solution to Quick check
1. (a) An exothermic reaction is a reaction which gives out heat energy to the
surroundings.
(b) An endothermic reaction is a reaction which absorbs heat energy from the
surroundings.
2. Enthalpy change is the amount of heat given out or taken in during a reaction.
The symbol used is ΔH.
3. (a) Endothermic (b) Exothermic
4. (a) The reaction is exothermic because ΔH is negative which means that heat
energy is lost from the reactants.
(b) Amount of heat evolved = 2 x 57.3 = 114.6 kJ
5. (a) decomposing zinc carbonate: endothermic
(b) reacting potassium with water: exothermic
(c) forming hydrogen atoms: endothermic
(d) forming chlorine molecules: exothermic
(e) respiration: exothermic
(f) photosynthesis: endothermic
Return
Enthalpy Changes

19. Chapter 13
Enthalpy Changes
To learn more about energy changes in
Chemical Reactions, Click on the links below!
1. http://www.ausetute.com.au/enthchan.html
2. http://www.avogadro.co.uk/h_and_s/enthalpy.htm